The present invention relates to an image signal processing method for a high speed digital facsimile apparatus and, more particularly, to a halftone processing method which allows a receiver to clearly reproduce in binary picture elements, i.e., black and white, those images of middle tones or halftones sent thereto from a transmitter.
A typical method for the reproduction of halftone images in black and white picture elements is the dither method. In this method, the density level S(i,j) of each picture element (i,j) constituting a halftone image is compared with a corresponding threshold component T.sub.D (k,l) of a threshold matrix (dither matrix) T.sub.D, which has M.times.N threshold components. If the density level S(i,j) is larger than or equal to the threshold component T.sub.D (k, l), the picture element will have (logical) "1" or black density level when reproduced; if not, then (logical) "0" or white density level.
The dither method is applicable to a high speed digital facsimile apparatus to reproduce at a receiver a halftone image transmitted thereto. Usually, for such an application, sixteen successive density levels are predetermined from the white level to the black level, i.e. threshold levels or slice levels "0" to "15". A plurality of threshold levels are selected out of the predetermined sixteen threshold levels at random and arranged in the horizontal and vertical scan directions to form a threshold pattern, which corresponds to the dither matrix. A CCD image sensor of a scanner produces an analog signal whose density levels correspond to individual picture elements are compared with the threshold levels in the threshold pattern in one to one correspondence, whereby each of the picture elements in the analog signal is sorted into the black or white level. This method can effectively reproduce a halftone image which resembles one on a document transmitted, because the picture elements share as many as sixteen successive threshold levels.
As described above, the density levels of the picture elements are compared with the threshold levels in one to one relation. In otherwords, the threshold level differs from one picture element to another at least in the horizontal scan direction; the threshold level is changed for every other picture element. This brings about a problem, however. When the resulting binary signal is coded by the ordinary modified Huffman system to be transmitted, the black or white run lengths will become shortened unless the halftone is extremely close to black or white. The shorter run lengths would degrade the coding efficiency and, thereby, increase the time period necessary for transmission.